Any of the following three types of materials; wetted explosives that when dry are explosives of Class 1 (other than those of compatibility group A) or are specifically authorized by name in the Hazardous Materials Table; self-reactive materials that are liable to undergo, at normal or elevated temperatures, a strongly exothermal decomposition caused by excessively high transport temperatures or contamination; or readily combustible solids that may cause a fire through friction, show a burning rate faster than 2.2 mm (.087") per second, or be ignited and react over the whole length of a sample in 10 minutes or less.

Note: Labels used to identify a subsidiary risk must have the class number obliterated.

Also see:

What are the Federal Regulations for Hazardous Materials Labeling?